[panda_description] Update inertials from upstream franka_description Gazebo URDF

Created the Gazebo URDF from upstream franka_description package and
then manually merged the inertials and new TCP frame into our packaged
URDF. Should fix #122

Note: There are two types of URDFs in the upstream repository. One also
has simplified, larger collision shapes. Since we possibly want to
simulate using the URDFs here, I kept the configuration using the convex
meshes rather than primitive shapes.